1. Field Of The Invention
The present invention relates to a die useful in hot hydrostatic extrusion of aluminum and aluminum alloys.
2. Description Of The Prior Art
As is well known, it is common practice to form aluminum and aluminum alloys into end products by use of a non-lubricating extrusion process such as direct or indirect extrusion in a temperature range between 350.degree. C. and 500.degree. C. In these extrusion processes, a specifically designed die such as a bridge die or a port-hole die has heretofore been used for performing extrusion of pure aluminum and Al-Mg alloys into end products having various structural profiles, including simple or complex profiles at a relatively high speed. If, however, a high tensile strength aluminum alloy such as Al-Cu alloy is extruded at such a high speed, the end products are formed with many cracks even when the end products have simple profiles. Thus, the extrusion speed must be limited to a lower level, viz., 2 to 3 m/min. The cracks are caused primarily by a partial temperature rise in the products due to friction between the die surface and the products, as well as by lack of uniformity in the strain and stress distributions caused by the flat die. It will thus be seen that while the conventional direct or indirect extrusion of aluminum or aluminum alloys has reached a highly skilled level there still remain many unsolved problems caused by the friction inherent in such non-lubricating extrusion processes.
On the other hand, the hydrostatic extrusion process, which is a most excellent lubricating extrusion technique, eliminates these problems. In this process, a pressure medium consisting of a material having a high lubricity and good pressure transmittance is introduced into a container and provides a fluid film between the billet and the die thereby reducing friction therebetween. Thus, a high tensile strength aluminum alloy such as an Al-Cu alloy can be extruded in a cold state at a high speed of several hundreds of meters per minute.
The present inventors have conducted many experiments involving hydrostatic extrusion, both hot and cold, of various materials in order to improve the workability of the process.
From these experiments, the existence of several surprising phenomena have been discovered in the hot hydrostatic extrusion process, particularly when aluminum and aluminum alloys are extruded. Namely, the extrusion pressure is determined by the flow stress of the billet material and the extruding ratio. Consequently, it is possible to increase the extruding ratio using the same extruding pressure if metals such as copper, aluminum and aluminum alloys are extruded in the hot hydrostatic extrusion process because the flow stress of the copper and aluminum is decreased as the extruding temperature is increased. Experiments have been conducted using particular materials such as copper, aluminum and aluminum alloys having the property that the flow stress decreases with increasing extruding temperature.
These experiments have revealed that non-sticky materials such as copper can be hot extruded even when the extrusion is performed by using a conical die similar to that used in the cold hydrostatic extrusion process. However, it is impossible to apply the same die for extruding aluminum and aluminum alloys without the occurrance of many seizures and dead metal areas. Consequently, there still exists a need for a hot extrusion die which allows for satisfactory extrusion of aluminum and aluminum alloys.